Membrane electrode assemblies (MEAs) constituting solid polymer electrolyte fuel cells include electrode catalyst layers including a catalyst such as platinum and an ionomer. Since the catalyst such as platinum is expensive, reduction in the amount thereof to be used is required. Unfortunately, the reduction in the amount of the catalyst tends to deteriorate the battery performance. In order to overcome this disadvantage, an attempt has been made to improve the oxygen permeability of an ionomer constituting an electrode catalyst layer delivering oxygen sufficiently in an electrode.
Patent Literature 1 discloses a polymer electrolyte having high oxygen permeability and suitable as an ionomer for a cathode-side catalyst layer. The polymer electrolyte includes an asymmetric hydrophobic cyclic structure and a hydrophilic structure that contains a proton-conducting group and that is bonded to the asymmetric cyclic structure, and is usable as an ionomer for a cathode-side catalyst layer.
Patent Literature 2 discloses a membrane electrode assembly for a solid polymer electrolyte fuel cell that achieves high power generation characteristics under high-temperature and low- or non-humidified operating conditions. The membrane electrode assembly includes a cathode-side catalyst layer that contains a polymer (H) as a solid polymer electrolyte polymer. The polymer (H) has an ion exchange capacity of 0.9 to 2.5 mEq/g (dry resin), an oxygen permeability coefficient of 1×10−12 (cm3 (Normal)·cm/cm2·s·Pa) or more, which is measured at 100° C. by the high-vacuum method, and an oxygen/nitrogen separation factor of 2.5 or higher at 100° C. The polymer (H) contains a repeating unit (A) that includes a cyclic structure and no ion exchange group or precursor group thereof, and/or a repeating unit (B) that includes a cyclic structure and an ion exchange group or precursor group thereof, and the sum of the proportions of the repeating unit (A) and the repeating unit (B) is 20 mol % or more in all of the repeating units in the polymer (H).
Patent Literature 3 discloses a solid polymer electrolyte material excellent in ionic conductivity, water repellency, and gas permeability. The solid polymer electrolyte material contains a copolymer that contains a repeating unit based on a fluorine-containing monomer that gives a polymer having a main chain with an aliphatic ring structure by radical polymerization and a repeating unit based on a fluorine-containing vinyl compound represented by the formula: CF2═CF(R2)jSO2X.
Patent Literature 4 discloses a gas diffusion electrode excellent in electrode characteristics for oxygen reduction reaction. The gas diffusion electrode is a porous gas diffusion electrode with a catalyst layer containing a catalyst and a fluorine-containing ion exchange resin. The catalyst layer further contains a polymer compound having an oxygen permeability coefficient of 5×10−11 [cm3 (Normal)·cm/cm2·s·Pa] or higher and containing substantially no ion exchange group. The polymer compound contains a polymer including a repeating unit based on a perfluorocarbon with an aliphatic ring structure.
Patent Literature 5 discloses ionically conductive compositions useful in fuel cells, electrolysis cells, ion exchange membranes, sensors, electrochemical capacitors, and modified electrodes. The ionically conductive compositions contain ionomers containing polymerized units of monomers A and monomers B, wherein the monomers A are perfluoro dioxole or perfluoro dioxolane monomers, and the monomers B are functionalized perfluoro olefins having fluoroalkyl sulfonyl, fluoroalkyl sulfonate or fluoroalkyl sulfonic acid pendant groups or CF2═CF(O)[CF2]nSO2X.
Patent Literature 6 discloses a solid polymer electrolyte fuel cell which includes gas diffusion electrodes containing a catalyst and an ion exchange resin and serving as a fuel electrode and an air electrode. The fuel electrode is disposed on one side of a film-like solid polymer electrolyte and the air electrode is disposed on the other side thereof. The ion exchange resin contained in the air electrode contains a copolymer containing a polymerized unit A, a polymerized unit B, and a polymerized unit C. The details of the polymerized unit s are: the polymerized unit A: a polymerized unit based on tetrafluoroethylene; the polymerized unit B: a polymerized unit based on perfluorovinyl ether containing a sulfonic acid group; and the polymerized unit C: a polymerized unit based on perfluorovinyl ether including no ion exchange group or precursor group hereof.